Conventionally, cellulose acylate films, which are excellent in toughness and flame resistance, have been used as photographic substrates and various optical materials. In particular, recently, they are frequently used as optical transparent films for liquid crystal display devices. Cellulose acylate films, which are excellent in optical transparency and also in optical isotropy, are particularly advantageous as optical materials for use in devices handling polarized light such as liquid crystal display devices, and have been used as polarizer-protecting films and also as substrates for optical compensation films for widening view angle (view angle compensation).
A polarizing plate, a component of liquid crystal display device, has a polarizer and a polarizer-protecting film bonded to at least one side of the polarizer. Common polarizers are prepared by dyeing an oriented polyvinylalcohol (PVA)-based film with iodine or a dichroic dye. Frequently, a cellulose acylate film, in particular a triacetylcellulose film, that can be directly bonded to PVA is used as the polarizer-protecting film. The properties of the polarizing plate are significantly dependent on the optical properties of the polarizer-protecting film.
On the other hand, because cellulose acylate films generally show a moisture permeability drastically increased under high-temperature and high-humidity environment, leading to deterioration in properties, compared to other common substrates such as polyethylene terephthalate films, compounds called plasticizers are often added to the films. Typical examples of the plasticizers added to the cellulose acylate film include phosphoric triesters such as triphenyl phosphate and biphenyl diphenyl phosphate, phthalic esters, and the like (e.g., JP-A-9-95557 (“JP-A” means unexamined published Japanese patent application)). Other known examples thereof include sulfonamide compounds such as N-ethyl-toluenesulfonamide (e.g. JP-A-2004-315613), and it is known that increase in the plasticizer content leads to reduction of the moisture permeability and improvement of the film properties.
However, increase in the plasticizer content caused a problem of significant lowering of the glass transition point of the cellulose acylate film and deterioration in dimensional stability by film softening. In addition, simple increase in the addition amount of plasticizer unfavorably leads to incompatibility of the plasticizer with the cellulose acylate, and consequently to whitening of the resulting film and increase in optical anisotropy (e.g., retardation value in the thickness direction Rth). In addition, low-molecular weight plasticizers have high thermal volatility because of their low molecular weights, and they vaporize, for example, in the drying step during production. This problem has been considered as the major element involved in process contamination.
When a cellulose acylate film is used as an optical material, the optical anisotropy of the film is preferably lower in some applications. Among the so-called plasticizers described above, there are some compounds that are known to decrease the optical anisotropy of the cellulose acylate film, and fatty acid esters and amide compounds of particular polyvalent alcohols are disclosed as examples of such compounds (see e.g., JP-A-2001-247717, JP-A-2000-63560, JP-A-11-246704 and JP-A-2006-30937). However, increase in the addition amount of such a plasticizer often results in deterioration in compatibility thereof and causes whitening defects of the film. Further, there is a problem of process contamination caused by thermal volatilization during production. There is also a problem of elution of the plasticizer into the saponification solution used during processing of the polarizing plate.
For example, to solve the problems due to thermal volatility described above, known are methods of obtaining a cellulose acylate film lower in the moisture permeability by controlling the volatility by using polymeric compounds such as rosin resin, epoxy resin, ketone resin, or toluenesulfonamide resin (e.g., JP-A-2002-146044). However, the polymeric plasticizers are not sufficiently compatible with the cellulose acylate, and are not compatible as much as the low-molecular weight plasticizers.
Therefore, there exists an urgent need for development of a modifier that is favorably compatible with cellulose acylate, resistant to volatilization or precipitation during casting and drying, and resistant to elution into the saponification solution, and that gives a cellulose acylate film lower in optical anisotropy.
For example, citric esters were proposed as such (e.g., JP-A-11-092574), but they still had problems in that they are insufficient in lowering optical anisotropy and the dope stability was insufficient.